1. Field of the Invention
The present invention relates to integrated circuits and in particular to power output stages utilizing vertical PNP transistors with isolated collector.
2. Description of the Prior Art
In integrated circuits driving external inductive and/or capacitive loads by means of output power stages, a saturation condition may take place whereupon the collector of an output transistor cannot deliver to the external load the whole current as driven through the base (i.e. Ic=.beta. Ib). A situation of this kind may happen for example when the collector of the output transistor is disconnected from the load or when the load is an inductance uncapable of taking the necessary current.
Furthermore, if the output stage is made using a vertical PNP transistor having a collector isolated from ground (from the substrate), the collector current of the transistor which is not taken by the load flows to ground (i.e. is injected into the substrate of the integrated circuit) through a parasitic PNP transistor whose structure is intrinsic to the structure of a vertical PNP transistor with isolated collector. This phenomenon is origin of remarkable problems due to:
power dissipation with a consequent increase of the operating temperature of the device; PA1 passage of current through the substrate with attendant probabilities of raising the ground potential (dirtying the ground of the device); PA1 undue current drainage during the switching-on of the integrated circuit.
According to a generally known technique, more or less sophisticated circuits are employed which intervene when the output PNP transistor enters saturation in order to prevent a condition of an excessively deep saturation thereof. The design of these prior art circuits has remarkable critical aspects which must be considered during the designing in order to ensure an adequate precision of the intervention threshold of the antisaturation circuit, besides a relatively large area is necessary for implementing these circuits within the integrated circuit.